Choosing an Energy Filter

Introduction

In many cases, the threads recommend that you filter ACIS event files on
energy before beginning the analysis. See
the Filtering Data and
Analysing the ACIS Background with the "Blank-Sky" Files threads for examples. Be aware
that the energy column in the event file only represents the nominal
energy of the incoming photon. This is fine for global energy
filtering, as shown here, but any more detailed analysis
involves the use of an RMF;
see the CIAO Threads for more information.

Determining the Filter

There are several things to consider when choosing an energy filter
for your data. Four major concerns are outlined below with
sample filters for illustrative purposes. Depending on
your analysis needs, it may be necessary to combine several of
the following examples into one energy filter.

Calibration
[energy=300:10000]

The energy scale of the ACIS CCDs is only calibrated over the range
0.277-9.886 keV.

The detectors were calibrated on the ground using various energies
between 0.277 keV (the carbon K line) and 9.886 keV (the germanium K line).
In flight, there are fewer energies and a narrower energy range with
which to calibrate the detectors: 1.5-5.9 keV for an onboard
radioactive calibration source, plus the oxygen
lines from E0102 at ~0.6 keV.
Therefore, it is not possible to extend the calibration beyond this range.

ACIS Background
[energy=300:7000]

The high energy particle background is fairly flat in the 2-7 keV
range. However, it both rises sharply below the 0.3 keV mark and
climbs by a factor of 8 between 7-10 keV. For more details, see
this
memo (PS, 17 pp) on characterizing the ACIS
background, specifically Figures 1 and 2.

If you are looking at the faint extended structure of a source, the
high background rate matters. Reducing the amount of background in
your image will help make the structure more prominent.

ACIS QE Contamination
[energy=500:7000]

There has been a continuous change in the
ACIS QE since launch.
This is most likely due to molecular contamination building up
on the cold optical blocking filter and/or the CCD chips.
This contamination is the most severe at low energies; above
1 keV, the contamination is less than 10% to date. Based on
the scientific analysis, the user must decide where to set the
lower limit of the energy filter (e.g. 0.5 keV vs 1.0 keV).
Full details are available from the ACIS
QE Contamination why topic.

Pileup
[energy=300:10000]
[energy=300:]

For unpiled sources, no source photons above 10 keV are
expected. This is due to the fact that the effective area of the telescope
is zero beyond that point. In this case, a cut at 10 keV is fine:
[energy=300:10000]. If a source is
piled,
however, two 6 keV photons could be detected as a single 12 keV
photon. If you are intending to fit a model to the piled
spectrum, do not provide an upper energy level for the filter:
[energy=300:]. Some pileup models require
all the high energies to be present in the input spectrum in
order to calculate the pileup fraction accurately.